1. Field of the Invention
The present invention relates to a microcomputer used in a system having an external storing unit and/or a peripheral unit such as an application specific integrated circuit.
2. Description of Related Art
An external storing unit (hereinafter, called an external memory) and a peripheral unit such as an application specific integrated circuit (ASIC) are generally used for a system in which a microcomputer is used. In this case, to perform a program of a central processing unit (CPU) of the microcomputer, the CPU gains access to data stored in the external memory or data of the ASIC.
In a system using the microcomputer, an execution rate for a program of the CPU considerably influences on the performance of the microcomputer. In other words, a reading speed of data from the external memory and a writing speed of data to the external memory influence on the performance of the microcomputer. Therefore, a type of external bus interface disposed in the microcomputer corresponds to a timing specification of the external memory, and it is not fundamentally required of the microcomputer having the external bus interface corresponding to a timing specification of the external memory to set a setup time or a hold time between signals. Therefore, an access time is given as sufficient as possible in case of the external bus interface corresponding to a timing specification of the external memory. Also, the microcomputer having another type of external bus interface can gain access to various types of ASICs, and it is required of the microcomputer having the external bus interface corresponding to a timing specification of the ASIC to sufficiently set a setup time and a hold time between signals.
In a prior art, an external bus timing of the microcomputer is set to the external bus timing corresponding to only one of the two types of external bus interfaces of the microcomputer. Therefore, in cases where a microcomputer is used in a system having an external memory and an ASIC in external areas, the performance of the microcomputer in an execution rate for a program deteriorates when the microcomputer gains access to the external memory, it is difficult for the microcomputer to gain access to the ASIC, or it is impossible for the microcomputer to gain access to the ASIC.
Also, in cases where a bus cycle is shortened due to the heightening of an operation speed of the microcomputer used in a system having an external memory and/or an ASIC, it is required to shorten a rise time or a fall time of a signal output from the microcomputer to an external memory or an ASIC. Therefore, a driving performance of an output driver of the microcomputer is set to become stronger.
However, in cases where an output impedance of the output driver is lower than a characteristic impedance of a substrate on which the external memory and/or the ASIC are displaced with the microcomputer, ringing occurs due to the reflection of a signal between the microcomputer and the external memory or between the microcomputer and the ASIC, the system including the microcomputer is erroneously operated, and radiation noise occurs in the system.
Also, the characteristic impedance of the substrate is changed in dependence on the system. Therefore, there is a case where the output impedance of the output driver of the microcomputer differs from the characteristic impedance of the substrate when the output impedance of the output driver of the microcomputer is fixed. In this case, ringing or signal delay occurs due to the difference between the output impedance of the output driver and the characteristic impedance of the substrate.
As is described above, because the conventional microcomputer has the above-described configuration, in cases where the conventional microcomputer is used in a system having the external memory and the ASIC in external areas, a problem has arisen that the performance of an execution rate of the microcomputer deteriorates.
Also, in cases where the output impedance of the output driver of the microcomputer differs from the characteristic impedance of the substrate in a system having the external memory and/or the ASIC in external areas or an external area, a problem has arisen that the performance of an execution rate of the microcomputer deteriorates.